This invention is in the field of elastic wave generation, transmission and detection in the earth. More particularly, it is concerned with determining the position in the earth of the drill bit during a drilling operation in a deep borehole. Still more particularly it concerns determination of the velocity of elastic waves in the earth.
In the prior art, various means have been devised for determining the position of the borehole in North-South, East-West coordinates, at selected depths during a drilling operation. This has been done by using survey instruments specially designed for introduction into the borehole and sometimes introduced through the drill pipe, which by their internal mechanism make a record of the slope (or angle with the vertical) of the borehole at each of a plurality of selected depths and a measure, in relation to the magnetic compass, of the azimuth of the slope of the borehole. Knowing the direction of slope and magnitude of slope, at each of a plurality of selected known depths and assuming that the borehole is straight in between the survey points at which measurements are made, the profile of the hole in three dimensions can be plotted.
The disadvantage of this particular system is that it requires a stoppage of the drilling process so that the drill pipe can be broken and the survey instrument inserted on a wire line into the drill pipe. Since the cost of operation of the drill rig runs into many hundreds of dollars a day, time lost from the drilling operation, to make these measurements, is extremely expensive. There is understandable reluctance on the part of drillers to make these necessary measurements, so that at some future time, when this knowledge is needed, it will not be available and cannot be easily obtained.